1. Field of the Invention
This invention relates generally to a start-up heater for a fuel cell system and, more particularly, to a start-up heater for a fuel cell system, where the heater is coupled to a cold plate that warms the cooling fluid and fuel cell stack at system start-up.
2. Discussion of the Related Art
Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. The automotive industry expends significant resources in the development of hydrogen fuel cells as a source of power for vehicles. Such vehicles would be more efficient and generate fewer emissions than today's vehicles employing internal combustion engines.
A hydrogen fuel cell is an electrochemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is disassociated in the anode, typically by a catalyst, to generate free hydrogen protons and electrons. The hydrogen protons pass through the electrolyte to the cathode. The hydrogen protons react with the oxygen and the electrons in the cathode, typically by a catalyst, to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode. The work acts to operate the vehicle.
Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell for vehicles. The PEMFC generally includes a solid polymer electrolyte proton conducting membrane, such as a perfluorinated acid membrane. The anode and cathode typically include finely divided catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with an ionomer. The combination of the anode, cathode and membrane define a membrane electrode assembly (MEA). MEAs are relatively expensive to manufacture and require certain conditions for effective operation. These conditions include proper water management and humidification, and control of catalyst poisoning constituents, such as carbon monoxide (CO).
Many fuel cells are typically combined in a fuel cell stack to generate the desired power. For example, a typical fuel cell stack for a vehicle may have two hundred stacked fuel cells. The fuel cell stack receives a cathode input gas as a flow of air, typically forced through the stack by a compressor. Not all of the oxygen in the air is consumed by the stack and some of the air is output as a cathode exhaust gas that may include water as a stack by-product. The fuel cell stack also receives an anode hydrogen input gas that flows into the anode side of the stack.
The membranes in the fuel cell stack must have a certain wetness or humidity level for proper stack operation. When the fuel cell system is turned off in a sub-zero temperature environment, any remaining water within the stack generally freezes. When the fuel cell system is turned back on, it will not be able to immediately generate the desired output power because the water in the stack is frozen and the stack is so far below its operating temperature. Therefore, starting a fuel cell system from sub-zero temperatures is a significant problem in a fuel cell system design.
Currently, it is known to selectively switch the output of the fuel cell stack to a resistor bank that operates as a temporary load during system start-up. The stack will gradually increase its operating temperature through the stack inefficiencies, i.e., the plate-to-plate resistances, over time as the fuel cell operates and the output power is being dumped to the resistor bank. However, this process takes a relatively long time, and thus improvements to fuel cell system start-up at sub-zero temperatures are needed.